For many internal combustion engines, their engine control strategy has three important parameters: spark timing (or fuel injection timing in a diesel engine), the exhaust gas recirculation (EGR) rate and the air/fuel ratio (AFR).
To implement EGR, a fraction of the exhaust gas is recycled from the exhaust system back to the intake system. The recirculated exhaust gas is mixed with the fresh fuel-air mixture before entering the cylinders. EGR can be implemented in internal and external configurations. External configurations can be implemented as either a high pressure or low pressure loop, depending on whether the recirculated exhaust is introduced to the intake post compressor (high pressure loop) or pre-compressor (low pressure loop).
EGR has a long history of use in both diesel and spark-ignited engines for reduction of NOx emissions. It affects combustion in several ways. The combustion is cooled by the presence of exhaust gas, that is, the recirculated exhaust gas absorbs heat. The dilution of the oxygen present in the combustion chamber reduces the production of NOx. Also, if exhaust gas is being recirculated, less air is breathed by the engine, so reducing the amount of exhaust gas produced.
As a relatively simple and low cost technology, EGR can be expected to be widely used on all types of engines. Of particular interest is that EGR can reduce the need for fuel enrichment at high loads in turbocharged engines and thereby improve fuel economy.